The present invention relates generally to diagnostic imaging and, more particularly, to a method and system of calibrating an x-ray detector to reduce artifacts in an image reconstructed from x-ray attenuation imaging data acquired by the x-ray detector.
X-ray imaging is a non-invasive technique to capture images of medical patients for clinical diagnosis as well as inspect the contents of sealed containers, such as luggage, packages, and other parcels. Increasingly, x-ray systems are being used to non-invasively search and/or screen airplane, boat, and train passengers as well as entrants to court buildings, government offices, sporting events, concerts, and other venues of heightened security interest. To capture these images, an x-ray source irradiates a scan subject with a fan beam of x-rays. The x-rays are then attenuated as they pass through the scan subject. The degree of attenuation varies across the scan subject as a result of variances in the internal composition of the subject. The attenuated energy impinges upon an x-ray detector designed to convert the attenuating energy to a form usable in image reconstruction. A control system reads out electrical charge stored in the x-ray detector and generates a corresponding image. For a conventional, screen film detector, the image is developed on a film and displayed using a backlight.
Increasingly, flat panel, digital x-ray detectors are being used to acquire data for image reconstruction. Flat panel detectors are generally constructed as having a scintillator, which is used to convert x-rays to visible light that can be detected by a photosensitive layer. The photosensitive layer includes an array of photosensitive or detection elements that each store electrical charge in proportion to the light that is individually detected. Generally, each detection element has a light sensitive region and a region comprised of electronics to control the storage and output of electrical charge. The light sensitive region is typically composed of a photoconductor, and electrons are released in the photoconductor when exposed to visible light. During this exposure, charge is collected in each detector element and is stored in a capacitor situated in the electronics region. After exposure, the charge in each detector element is read out using logic controlled electronics.
Each detector element is conventionally controlled using a transistor-based switch. In this regard, the source of the transistor is connected to the capacitor, the drain of the transistor is connected to a readout line, and the gate of the transistor is connected to a scan control interface disposed on the electronics in the detector. When negative voltage is applied to the gate, the switch is driven to an OFF state, i.e. no conduction between the source and drain. On the other hand, when a positive voltage is applied to the gate, the switch is turned ON resulting in connection of the source to the drain. Each detector element of the detector array is constructed with a respective transistor and is controlled in a manner consistent with that described below.
Specifically, during exposure to x-rays, negative voltage is applied to all gate lines resulting in all the transistor switches being driven to or placed in an OFF state. As a result, any charge accumulated during exposure is stored in each detector element capacitor. During read out, positive voltage is sequentially applied to each gate line, one gate at a time. In this regard, only one detector element is read out at a time. A multiplexer may also be used to support read out of the detector elements in a raster fashion. An advantage of sequentially reading out each detector element individually is that the charge from one detector element does not pass through any other detector elements. The output of each detector element is then input to a digitizer that digitizes the acquired signals for subsequent image reconstruction on a per pixel basis. Each pixel of the reconstructed image corresponds to a single detector element of the detector array.
As described above, indirect detection, digital x-ray detectors utilize a layer of scintillating material, such as Cesium iodide (CsI), to convert incident radiation to visible light that is detected by light sensitive regions of individual detector elements of a detector array. Generally, the transistor controlled detector elements are supported on a thin substrate of glass. The substrate, which supports the detector elements as well as the scintillator layer, is supported by a panel support. The panel support is not only designed to support the detector components, but also isolates the electronics that control the detector from the image detecting components. The electronics are supported by the panel support and enclosed by the back cover.
Conventional flat panel detectors have utilized a chiller to regulate the temperature of the detector to be within a given temperature range. The chiller, however, adds to the size, weight, and complexity of the detector. As consumer demand for lighter and more maneuverable x-ray detectors has increased, it has become necessary to design x-ray detectors that do not use a chiller to actively control detector temperature. Specifically, increasingly, flat panel detectors are cooled using natural convection. Since natural convection is a passive means of regulating temperature, it can be difficult to timely maintain detector temperature during heavy detector usage periods. That is, as the number of data acquisitions increases within a given time period, so does the thermal load placed on the x-ray detector. If the intervals between data acquisitions are short, the x-ray detector may not sufficiently cool between data acquisitions to maintain a desired thermal equilibrium. As such, the x-ray detector may be placed into use despite the x-ray detector having a non-ideal temperature or a temperature outside a desired temperature range.
As a result of the acquisition of data when the detector is outside the desired temperature range, image quality is affected. That is, during the image reconstruction process, a number of detector calibrations or data corrections are typically applied to improve image quality. These calibrations and corrections usually assume a given temperature for the x-ray detector during data acquisition. Specifically, the calibration and corrective process typically assumes that the x-ray detector has a temperature within a desired temperature range during the acquisition of imaging data. As such, the calibration and corrective processes do not take into account whether the x-ray detector temperature during data acquisition was outside the presumed temperature range. Moreover, the calibration and corrective measures generally ignore the specific temperature of x-ray detector even when the temperature is within the desired range. By failing to take into account the specific temperature of the x-ray detector or that the temperature is outside a predefined temperature range, the calibration and corrective measures may inadequately or improperly applied.
Also affecting the accuracy and precision of x-ray data calibration that is particularly applicable for offset correction is the time interval between the acquisition of x-ray imaging data and the acquisition of offset imaging data. Offset correction is a calibration process whereby an offset image is acquired without x-rays and used to mask acquired imaging data. The theory behind the offset correction technique is that artifacts attributable to the x-ray detector or scan environment itself may be masked from the imaging data acquired with x-rays. In application, the acquired offset data is used to reconstruct an offset image that is subtracted from the imaging data image. This results in a subtraction of the artifacts in the imaging data image that are also present in the offset image and, thus, improves image quality. As a result of the time differences between the acquisition of imaging data and offset data, power transients may introduce artifacts that are not removed in the subtraction process.
It would therefore be desirable to have a process of calibrating acquired x-ray attenuation data that takes into account the specific temperature of the x-ray detector during data acquisition. It would also be desirable to have a calibration technique that reduces the effect of power transients in an offset image used for offset calibration of imaging data.